Step by step
The question
Question: 60 patients from a clinic volunteered for the study.
Half of them were chosen at random to do a daily stretch.
The stretch group had less back pain.
What can the study say?
There are four choices.
First we solve it, the normal way.
Then we pick.
1 Two questions
To judge a study, we ask two questions.
How were the patients picked?
And how were the groups made?
2 How they were picked
The story says the patients volunteered.
They chose to join.
So: no, not at random.
People who join by themselves can be different from the others.
Careful!
A big group is not enough.
Volunteers may be different from the rest of the clinic.
So what we find is true only for people like these, not for the whole clinic.
3 How the groups were made
The patients were chosen at random to do a daily stretch.
So: yes.
Chance made the groups, not the patients.
So the two groups are alike, except for the stretch.
So the difference comes from the stretch: the stretch causes it.
4 What the study can say
So the study can say: the stretch causes this, but only for patients like these.
5 Check
Let's check with the story.
The first sentence says: volunteered.
That matches.
The second sentence says: chosen at random to do a daily stretch.
That matches too.
So the answer is right!
6 Pick the choice
Our answer is causes it, for people like these.
That is choice B.
7 Why not the others
A is causes it, for the whole clinic.
Wrong: volunteers are not a random sample: only for people like these.
C is linked, for people like these.
Wrong: the groups were made at random, so the stretch causes it.
D is linked in the clinic, maybe not caused.
Wrong: the groups were made at random, and volunteers do not speak for the whole clinic.
Let's look back.
First: two questions.
Then: how they were picked.
Then: how the groups were made.
Then: what the study can say.
Then: check.
Then: pick the choice.
Last: why not the others.